Database systems are evolving from disk-based single server systems with high input/output (I/O) rates and small memories, to clusters of independent nodes which hold the database entirely in memory. These systems often rely on horizontal partitioning (or sharing) for scaling and k-safety (synchronous replication) to provide durability.
Data contention and metadata contention are ongoing challenges for online transaction processing (OLTP) systems. The probability of contention increases as the number of parallel transactions in the system increases. Transaction response time is linked to the number of active parallel transactions by Little's law. This law states that the number of transactions in a system equals the throughput multiplied by the response time.